1. Field of the Invention
The invention relates to the preparation of novel, broad host spectrum nodulation signals (Nod factors).
2. Description of the Related Art
Sol bacteria which belong to the genera Azorhizobium, Bradyrhizobium, Sinorhizobium and Rhizobium, (which are referred to under the general term rhizobia) are capable of interacting with the roots of legumes in order to form nodules in which they fix atmospheric nitrogen. However, only certain combinations of bacteria and plants result in nodulation and host specificity of rhizobia varies greatly [LONG, Cell. 56, 203 (1989)]; [MARTINEZ et al., Crit. Rev. Plant Sci., 23, 483 (1990)]; [DENARIE et al., in Molecular Signals in Plant-Microbe Communications, D.P.S. Verma Ed. pp. 295-324 (CRC Press, Boca Raton, 1992)]. Certain rhizobia (for example R. leguminosarum and R. meliloti) form nodules on only a small number of legume species, while, on the other hand, others have a broader host spectrum and can form an association with a large number of plants.
Nodule formation results from a coordinated expression of plant genes and bacterial genes. The expression of rhizobial nodulation genes (nod) is controlled by nodD regulator genes whose products are activated by flavonoids which are secreted by the roots of the plants. The ability of the NodD proteins to interact with the plant flavonoids in a specific manner defines a first level of host specificity.
Moreover, two categories of structural nod genes exist: genes which are in common and specific genes. The nodABC genes are common to all rhizobia, while nod genes, which are specific to the species, are the major determinants of host specificity.
It has been shown that the common nod genes and the specific nod genes are simultaneously involved in the production of extracellular Nod factors which cause deformation of root hairs in legumes. Some inventors have identified Nod factors, termed NodRm, in R. meliloti which factors have a lipo-oligosaccharide structure, whose biosynthesis is under the control of common nodABC genes, and which are glucosamine oligomers linked to each other by .beta.-1,4 bonds, N-acylated on the non-reducing terminal glucosamine and N-acetylated on the other glucosamine residues (Application PCT FR/9100283 in the names of the INSTITUT NATIONAL DE LA RECHERCHE AGRONOMIQUE and the CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE). Host specificity is subsequently determined by the nature of the substituents attached to this skeleton which they have in common. In the case of R. meliloti, the function of major host specificity genes (nodH and nodPQ) is to determine the sulfation of these lipo-oligosaccharide factors [ROCHE et al., Cell, 67, 1131 (1991)], while, in the case of R. leguminosarum, the nodFE genes control the synthesis of a highly unsaturated lipid residue [SPAINK et al., Nature, 354, 125, (1991)].
The strain Rhizobium sp. NGR234 has a unique place amongst the legume symbionts; it has, in fact, the broadest host spectrum of all known rhizobia, and it is known at present that it causes nodulation of over 60 legume species. Amongst these hosts there are, in particular, most of the commercially important legumes such as, for example, soya bean or groundnut. Rhizobium NGR234 can, moreover, cause nodulation of plants which do not belong to the legumes, such as, for example, Parasponia andersonii.